The invention relates generally to apparatus and methods for using frictional drives including conformable rollers in electrostatography, and more particularly to the use of frictional drives for transferring toner images in electrophotography.
During the production of color images in an electrostatographic engine in general and in an electrophotographic engine in particular, latent images on photoconductive surfaces are developed by electrostatic attraction of triboelectrically charged colored marking toners. A latent image is created in a color electrophotographic engine by exposing a charged photoconductor (PC) using, for example, a laser beam or LED writer. Individual writing of each latent image must be properly timed so that the various toner images developed from the latent images can be transferred in registry. Each of these toner images corresponds to one of several color separations that will make up a final color image. The toned image separations must then be transferred, in register, to either a receiver or to an intermediate transfer member (ITM). The toner images can be transferred, either sequentially from a plurality of photoconductive elements to a common receiver in proper register, or transferred, sequentially, in proper register, to one or more ITMs from which all images are then transferred to a receiver. Alternately, each photoconductive surface may be associated with its own ITM, which transfers its toned image, in proper register with those of the other ITMs, to a receiver, for the purpose of enhancing the transfer efficiencies as described more fully in T. Tombs et al., U.S. Pat. No. 6,075,965. A toner image on the receiver is thermally fused in a fusing station, typically by passing the receiver through a pressure nip which includes a fuser roller and a pressure roller.
A key feature is that transfers must be performed in proper registry. The degree of misregistration that can be tolerated in an acceptable print depends on the image quality specifications. For high image quality color applications, allowable misregistration is typically less than 0.004 inch (0.1 mm) and preferably less than 0.001 inch (0.025 mm). Misregistration is often examined using 10xc3x97 to 20xc3x97 loupes to determine relative positions of interpenetrating fiducial line or rosette patterns. In systems involving elastomeric rollers and in particular in machines including compliant incompressible elastomeric rollers as intermediate transfer members as described by D. Rimai et al., U.S. Pat. No. 5,084,735, the rollers are known to deform as they roll under pressure against a photoconductive surface which may include a web or a drum. These intermediate transfer members also undergo deformations as they roll against receiver materials either as continuous webs or as cut sheets that can be supported by a web or by a backup roller assembly, or by combinations of these. Other prior art disclosing ITMs include U.S. Pat. Nos. 5,110,702; 5,187,526; 5,666,193 and 5,689,787.
Deformations of conformable members produce a phenomenon known as overdrive. Overdrive refers to the fact that in a nip including an elastomeric roller and a relatively rigid roller that roll without slipping, the surface speed of the rigid roller exceeds the surface speed of that portion of the elastomeric roller that is far from the nip. Far away from the nip means at a location where any distortions caused by the nip are negligible. The difference in peripheral speeds far from the nip is a result of the strains occurring in the elastomeric roller surface as it approaches and enters the nip.
The concept of overdrive may be better understood by referring to the sketches in FIGS. 1 and 2.
In FIG. 1a, a rigid cylindrical wheel or roller is driven without overdrive. In such an example, each point on the periphery has a velocity v0 given by the product of the angular velocity xcfx89 and the radius r of the roller, i.e., v0=xcfx89r.
In FIG. 1b, a deformable externally driven roller is illustrated. The deformation illustration is exaggerated to facilitate explanation of the concept that when a substantially incompressible compliant member is in a transfer nip, for example, a deformation will occur that causes the radius to be smaller in the nip area but to bulge out at pre-nip and post-nip areas. The dotted line shows the original circular rigid case of FIG. 1a for comparison. The relationship of v0=xcfx89r still holds true for points on the roller far from the nip area where there is no deformation. However, this relationship is not true for the points in the pre-nip, nip and post-nip areas. For the roller illustrated in FIG. 1b the speed of a point in the nip area has a higher magnitude than that far from the nip. The speed ratio of the roller surface in the nip divided by the speed at a point far from the nip area characterizes overdrive.
More particularly consider, for example, a conformable roller having an externally driven axle, frictionally driving with negligible drag a movable planar element having a nondeformable surface. If the external radius of the roller far from the nip is r and the peripheral speed of the roller far from the nip is v0, then the surface velocity Vnip of the distorted portion of the roller in nonslip contact with the planar surface is given by
vnip=xcexxcfx89r
where xcex is a speed ratio defined by
xcex=(vnip/v0).
As defined here, overdrive (or underdrive) is numerically equal to the absolute value of the speed ratio minus one. The value of xcex is determined principally by an effective Poisson""s ratio of the roller materials, such as produced by a roller including one or more layers of different materials, and secondarily, by the deformation geometry of the nip produced by the engagement. The Poisson ratios of high polymers, including elastomeric polymers which for practical purposes are almost incompressible, approach 0.5. The Poisson ratios for highly compressible soft polymeric foams approach zero. It has been shown by K. D. Stack, xe2x80x9cNonlinear Finite Element Model of Axial Variation in Nip Mechanics with Application to Conical Rollersxe2x80x9d (Ph.D. Thesis, University of Rochester, Rochester, N.Y. (1995), FIGS. 5-6 and 5-7, pages 81 and 83) that the value of Poisson""s ratio for xcex=1 is about 0.3 for a roller driving a rigid planar element. For values of Poisson""s ratio larger than about 0.3, the circumference of the roller distorted by the nip is greater than 2xcfx80r, producing overdrive of the planar element with respect to the roller, i.e., the surface speed vnip of the distorted portion of the elastomeric roller within the nip and hence that of the planar element is greater than v0 (i.e., xcex greater than 1). For values of Poisson""s ratio smaller than about 0.3, the circumference of the elastomeric roller distorted by the nip is less than 2xcfx80r, producing underdrive of the planar element with respect to the roller, i.e., the surface speed vnip within the nip is smaller than v0 (i.e., xcex less than 1). Conversely, if a nondeformable planar element frictionally drives, with negligible drag, a roller having a Poisson ratio less than about 0.3 and causes it to rotate, one may speak of overdrive of the roller with respect to the planar element because the surface speed of the driven roller far from the nip is faster than the speed of the planar element.
With reference to FIG. 2b, when a roller transfer member formed of an elastomer that has a Poisson ratio of about 0.45 to about 0.5 is driving a rigid planar element that is moving through a nip and there is no slippage between the roller and the rigid element, the rigid element will be overdriven relative to the speed of the roller far from the nip. Where the roller is formed of a compressible material (i.e., experiences relatively large volume reduction upon compression), such as a foam, the distortion of the roller may be such (see FIG. 2a) that the surface of the roller is contracted rather than stretched. Compare FIG. 2a with the example of the elastomeric roller of FIG. 2b having little or no volume change upon compression, with each roller shown in driving engagement with a rigid planar element. In the example of the highly compressible roller (relatively large volume change upon compression) of FIG. 2a, the rigid planar element such as a recording sheet may be subject to an underdrive condition.
For purpose of further illustration, FIG. 2c illustrates an exemplary apparatus, indicated by the numeral 5, which includes two counter-rotating rollers 1 and 2 forming a pressure nip 3. Far away from the nip, rollers 1 and 2 have peripheral speeds v1 and v2 respectively. Roller 2 is hard, and roller 1 is conformable, with roller 1 having a strained volume portion sketched by a cross-hatched region 4 in the vicinity of the nip (deformation of the surface of roller 1 is not depicted). Hereinafter, the terms xe2x80x9chardxe2x80x9d and xe2x80x9cnon-conformablexe2x80x9d are used interchangeably, and refer to materials for which the Young""s modulus is greater than or equal to 100 MPa. Consider that one of the axles P or Q is caused to rotate by the action of an external agent, such as for example a motor, and the other axle is rotated by nonslip friction in the nip. The externally rotated roller is a driving roller, while the other is a (frictionally) driven roller. There are four extreme cases to consider. Case 1: roller 1 is the driving roller, and region 4 is a substantially incompressible elastomer, whereupon as explained above the peripheral velocity v2 of roller 2 far from the nip is greater than the peripheral velocity v1 of roller 1 far from the nip, and roller 2 is said to be overdriven. Case 2: the same materials as Case 1, except that roller 2 is the driving roller and roller 1 is the driven roller, whereupon roller 1 is said to be underdriven. Case 3: roller is the driving roller, and region 4 is a compressible resilient foam, whereupon the peripheral velocity v2 of roller 2 far from the nip is smaller than the peripheral velocity v1 of roller 1 far from the nip, and roller 2 is said to be underdriven. Case 4: the same materials as case 3, except that roller 2 is the driving roller and roller 1 is the driven roller, whereupon roller 1 is said to be overdriven. It should be noted that it is common practice to use the term xe2x80x9coverdrivexe2x80x9d in a generic or nonspecific fashion where either overdrive or underdrive technically exists.
It may be understood that to produce a frictional drive involving a conformable roller, there is a xe2x80x9clockdownxe2x80x9d portion within the contact zone of the nip where there is substantially no slippage between the driving and driven members. Moreover, during the continual formation and relaxation of the pre-nip and post-nip bulges or deformations on the conformable roller as it rotates through the nip, there may also be locations in the contact zone of the nip where the surface velocities of the two surfaces in contact differ, i.e., there may be localized slippages. Such localized slippages may occur just after entry (i.e., before lockdown occurs) and just before exit of a transfer nip (i.e., after lockdown ceases). These pre-lockdown and post-lockdown slippages, if they happen, take place over distances which are small compared to the nip width, and occur in opposite directions inasmuch as they are related to the formation and relaxation of the pre-nip and post-nip deformations, respectively. In order to avoid confusion below, a frictional drive is hereinafter defined as being nonslip if a region exists in the nip (i.e., the lockdown region) wherein the coefficient of friction is sufficiently large to provide a continuous frictional driving linkage between the contacting members within the nip. This definition excludes any localized slippages that may occur in the contact areas near the entry and exit of the nip, because these localized slippages are in opposite directions and any effects on the drive produced by them effectively cancel. In other words, the frictional linkage in the xe2x80x9clockdownxe2x80x9d portion is the only factor of importance in determining a driving connection produced by the nip. Hereafter, the words xe2x80x9cnonslipxe2x80x9d, xe2x80x9cslipxe2x80x9d and xe2x80x9cslippagexe2x80x9d refer to an externally measured behavior of the members involved in the frictional drive, e.g., as described below in the specification of the present invention.
Two materials in contact in a pressure nip may have different thicknesses or different Poisson ratios, so that overdrive at their interface can cause squirming and undesirable stick-slip behavior. For example, when roller transfer members are used to make a color print, such behavior can adversely affect the final image quality, e.g., by causing toner smear or by degrading the mutual registration of color separation images. Moreover, variations in overdrive, which are referred to herein as xe2x80x9cdifferential overdrivexe2x80x9d can occur along the length of a pressure nip, such variations being caused, for example, by local changes in engagement, such as produced by runout, or by a lack of parallelism, or by variations of dimensions of the members forming a pressure nip, such as for example out-of-round rollers. A differential overdrive caused by runout, such as produced by a roller having a radius as measured from the axis of rotation that varies around the roller circumference, results in a speed ratio that fluctuates as the roller rotates.
Herein, the term engagement, in reference to a pressure nip formed between two members having operational surfaces, is defined as a nominal total distance the two members are moved towards one another to form the nip, starting from an initial undeformed, barely touching or nominal contact of the operational surfaces. In FIGS. 1a and 1b, for example, the engagement is the distance the axis of rotation of the roller is moved towards the rigid planar element from a nominal initial kissing position. In an example of two parallel rollers, the engagement is an initial separation of the two axes of rotation (defined by a nominal initial kissing position with neither roller distorted) minus the actual separation of the axes after the nip is formed.
During transfer of a toner image in an elastomeric nip exhibiting overdrive or underdrive, an image experiences a length change in the process direction. This change in length causes a distortion in the final image that is objectionable. Change in the writing speed of an electrostatic latent image can correct for overdrive in a simple single-color engine. In a color electrophotographic engine, however, high quality color separations preferably are properly registered to a spatial accuracy comparable with the resolution of the image. In a color electrophotographic engine including a plurality of color stations, proper registration can be achieved by having each color station behave exactly in the same manner with respect to image distortion, e.g. by using rollers made as identical as possible to each other. However, this is expensive and impractical.
Specifically, in order to produce proper electrophotographic images using techniques of the prior art, properties of rollers must not vary outside predetermined acceptable tolerances. The properties include acceptable runout, reproducible and uniform resistivity and dielectric properties, uniform layer thicknesses, parallelism of the members, and responses of the rollers to changes in temperature and humidity experienced during routine operation and machine warm-up. Rollers must also maintain their properties within tolerances during wear processes so that adverse effects are not experienced on the final images as a result of wear. If the effects of wear cannot be compensated, the components must be replaced.
A roller may have variations in the location of the roller surface relative to the roller center as a function of angle during rotation that is commonly known as xe2x80x9crunoutxe2x80x9d. Runout may be caused by out of round rollers or by improper centering of an otherwise round roller or both. Runout may vary along the length of a roller. Since the magnitude of the overdrive produced by a deformable roller depends on engagement, runout will temporally and spatially modify the engagement and overdrive during the production of a single image, producing distortions that are objectionable. Runouts of 0.001 inch (0.025 mm) can produce unacceptable registration problems, with runouts of less than 0.0002 inch (0.05 mm) needed to achieve acceptable registration based on measured sensitivity of overdrive to engagement.
Further, rollers used in these applications are made from polymers that can change dimension by absorption of moisture and can change dimensions due to temperature changes. These dimensional changes further complicate the registration of color separations if the changes are not the same in each of the color separation stations included in a color electrostatographic engine.
Methods based on the prior art to produce a workable electrophotographic engine with useful image quality require very expensive manufacturing processes to control the properties and dimensions of the elastomeric rollers.
What is needed is a method to alleviate or effectively eliminate image distortion caused by overdrive or underdrive phenomena. While this can be performed by expensive algorithms to the writing scheme using sensors to detect surface speeds of elements during writing and transfer, a much more cost-effective method is desired.
There are several disclosures in the prior art that relate to the peripheral speeds of rollers. T. Miyamoto et al., xe2x80x9cImage Forming Apparatus with Peripheral Speed Difference Between Image Bearing and Transfer Membersxe2x80x9d, U.S. Pat. No. 5,519,475 have mentioned this explicitly in their title but the entire disclosure of this patent is about the roughness characteristics of elastomeric surfaces. U.S. Pat. No. 5,519,479 teaches the use of peripheral speed differences between a photoconductive member and an intermediate transfer member (ITM) to reduce the apparent roughness of the surface. The patent notes transfers from the photoconductive members to transfer intermediates where there is a peripheral speed difference of 0.5% to 3%. Another patent, K. Tanigawa et al., xe2x80x9cImage-Forming Apparatus with Intermediate Transfer Memberxe2x80x9d, U.S. Pat. No. 5,438,398 also includes disclosure relating to peripheral speeds. In particular, embodiments 6 and 7 suggest that an intentional peripheral speed difference of 1% helps with xe2x80x9ccentral dropoutxe2x80x9d defects. The patent notes that transfers of images are intentionally provided with differences in peripheral speeds but no description is provided relative to overdrive or underdrive as described herein. Another reference is M. Yamahata et al., xe2x80x9cDrive Mechanism for an Electrophotographic Apparatus for Ensuring Equal Rotational Speeds of Intermediate Transfer Devices and Photosensitive Devicesxe2x80x9d, U.S. Pat. No. 5,390,010. This reference specifically addresses the behavior of web photoconductors (PCs) and web ITMs with the central idea to use the same drive motor to drive an intermediate transfer web drive roller which in turn drives the web drive roller of a photoconductive web. Thus, disturbances in surface speed of the ITM web, such as might be caused by engagement of a cleaning station, etc., would be transmitted to the PC web so that there would not be image degradation due to slippage. Yamahata et al. do not discuss how this would affect the writing of an image. There is no disclosure in this patent of transfers where a nip is formed by an elastomeric member and the problems of overdrive or underdrive as it affects image registration. It is clear that this reference addresses the problem of slippage of the ITM relative to the PC when such slippage is caused by disturbances of the system.
U.S. Pat. No. 5,790,930 discloses a means for correcting for misregistration between an image-carrying member and an intermediate transfer web due to variations in the length of the two members. It accomplishes this by means of forcing a periodicity in the drive speeds. It can achieve this by means of either two motors or a single motor.
U.S. Pat. No. 5,376,999 discloses a method of correcting for speed mismatches between a photoconducting element and an intermediate transfer web due to the stretching of that web arising from the tension applied to that web. The strains described in this patent occur outside the nip. The patent discloses allowing one member to slip with respect to the other where both members are driven. There is no discussion of an elastomeric intermediate transfer member in this patent. In an elastomeric intermediate transfer member, the distortions occur due to the presence of stresses applied normally to the surface of the elastomeric member in the nip rather than due to stresses applied parallel to the surface of the elastomeric member.
U.S. Pat. No. 5,966,559 discloses a method and apparatus for adjusting a transfer nip between a toner image bearing member and a transfer backup roller in order to accommodate receiver stocks having different thicknesses. A sensor senses a parameter related to the thickness of a receiver member prior to movement of the receiver into the transfer nip and an adjustment device adjusts the nip spacing in order to reduce or eliminate an impact of the receiver entering the nip. This patent does not teach the use of the adjustment device to control engagement in the transfer nip.
In electrostatography in general and, more particularly in electrophotography, the elimination of overdrive or underdrive in a conformable nip is desirable because overdrive and variations in overdrive can cause image defects such as misregistration of color separation images objectionable to the customer. There is a need to provide simple, inexpensive means to control or eliminate overdrive related registration artifacts.
The invention includes a method and apparatus to control image defects related to transfer of toner images in an electrostatographic machine, including defects such as misregistration associated with overdrive or underdrive and variations in overdrive and underdrive in a transfer station including a toner image bearing member. Specifically, an engagement between an operational surface of a conformable toner image bearing member and an operational surface of another member forming a transfer nip is adjusted using an engagement adjustment device to control an overdrive or underdrive associated with the nip. In one aspect of the invention, a transfer nip for transferring a toner image includes two rollers supported by parallel shafts coaxial with each roller, the shafts separated by a controllable distance of separation and the engagement in the nip being controllably adjustable by an engagement adjustment device to increase or decrease the distance of separation. In another aspect of the invention, a transfer system includes a first transfer nip formed by a primary image forming member roller having a coaxial supporting first shaft and an intermediate transfer member roller having a coaxial supporting second shaft separated from the first shaft by a first controllable distance of separation, and a second transfer nip formed by the intermediate transfer roller and a transfer backup roller, the transfer backup roller having a coaxial supporting third shaft separated from the second shaft by a second controllable distance of separation, wherein the engagement in each of the first and second transfer nips is separately and controllably adjustable by an engagement adjustment device to respectively increase or decrease the distance of separation between the first and second shafts and the distance of separation between the second and third shafts. Preferably, an engagement adjustment device used according to the present invention in a toner transfer station provides a preselected amount of overdrive or underdrive between a toner image forming member and a receiver member to which a toner image is transferred. A transfer system according to the present invention may have a steady state controlled overdrive or underdrive, including the possibility of zero overdrive.
In yet another aspect of the invention, an engagement adjustment device is employed to control an overdrive or an underdrive in a fusing station of an electrostatographic machine.